Using loop exit prediction to accelerate or suppress loop mode of a processor

ABSTRACT

A processor predicts a number of loop iterations associated with a set of loop instructions. In response to the predicted number of loop iterations exceeding a first loop iteration threshold, the set of loop instructions are executed in a loop mode that includes placing at least one component of an instruction pipeline of the processor in a low-power mode or state and executing the set of loop instructions from a loop buffer. In response to the predicted number of loop iterations being less than or equal to a second loop iteration threshold, the set of instructions are executed in a non-loop mode that includes maintaining at least one component of the instruction pipeline in a powered up state and executing the set of loop instructions from an instruction fetch unit of the instruction pipeline.

BACKGROUND

To enhance processing efficiency, modern processors sometimes employ aloop mode to execute program loops. In the loop mode, the processorretrieves and executes instructions of the loop from a loop instructionbuffer, rather than repeatedly retrieving the instructions of the loopvia an instruction fetch unit. The loop mode allows the processor toconserve resources by, for example, placing the instruction fetch unitor other portions of the processor in a low-power state while in theloop mode. However, conventional loop mode operation is inefficientunder some conditions. For example, the loop mode is typically exited asa result of the processor encountering a branch misprediction for theloop exit instruction. The branch misprediction causes the instructionpipeline of the processor to be flushed thereby consuming additionalprocessor resources and resulting in a power overhead. For relativelyshort instruction loops, the resources consumed by the pipeline flushcan exceed the resources conserved by operating in the loop mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings. The use of the same referencesymbols in different drawings indicates similar or identical items.

FIG. 1 is a block diagram of an instruction pipeline within a processorimplementing loop exit prediction in a low power state in a loop mode inaccordance with some embodiments.

FIG. 2 is a block diagram of the instruction pipeline of FIG. 1 with aloop exit predictor in a powered up state during a loop mode inaccordance with some embodiments.

FIG. 3 is a block diagram of the instruction pipeline of FIG. 1illustrating additional aspects of the loop exit predictor and a loopmode in accordance with some embodiments.

FIG. 4 is a flow diagram illustrating a method of employing loop exitprediction to identify relatively large loop iterations for a loop modein accordance with some embodiments.

FIG. 5 is a flow diagram illustrating a method of employing loop exitprediction to identify small loop iterations for a loop mode inaccordance with some embodiments.

DETAILED DESCRIPTION

FIGS. 1-5 illustrate techniques for employing loop exit prediction (LEP)at a processor to conserve processor resources associated with employinga loop mode. The processor includes a LEP unit that predicts the exit ofeach executing loop. Based on the prediction by the LEP unit, theprocessor implements one or more loop management techniques, includingdeclining to enter the loop mode for a relatively short loop, exitingthe loop mode before an indication of a branch misprediction orencountering a branch misprediction, and accelerating entry into theloop mode for a relatively large loop. Each of these techniques reducesthe amount of resources consumed by the processor to employ the loopmode, thereby enhancing processing efficiency.

To illustrate, in some embodiments, the processor employs the LEP unitto predict the number of iterations of each executing loop in a programflow. In response to the LEP unit indicating that the number ofiterations for the loop is below a specified threshold number of loopiterations, the processor suppresses entry into the loop mode for theloop. The processor thereby avoids entering the loop mode when resourcecosts of entering the loop mode exceed resource savings from executingthe loop in the loop mode.

In some embodiments, the processor employs the LEP unit while executinga loop in the loop mode to predict when the loop is expected to exit. Inresponse to the LEP unit indicating a predicted loop exit, the processorinitiates exiting of the loop mode such as fetching and filling aninstruction pipeline with one or more next instructions to execute uponexiting the loop. The processor therefore does not wait for a branchmisprediction to indicate the loop exit or trigger the loop exit, andthus this procedure avoids a pipeline flush that consumes processorresources and delays further instruction execution. LEP is used topredict loop exit branches even during the loop mode. In certainembodiments, a dedicated LEP unit within a processor performs LEP. SinceLEP is specifically tuned for loop exit branches, LEP accuracy is higherthan an accuracy of general branch prediction applied by one or morebranch predictors during execution of loops.

The processor also uses the predicted number of iterations provided bythe LEP unit to identify a relatively large loop, and the processoraccelerates entry into the loop mode ahead of executing the large loop.In particular, the processor nominally enters the loop mode in responseto a first threshold number of iterations of the loop being executed orlikely to be executed before entering the loop mode to make sure that aloop is actually encountered and successfully completing loops throughthe set of loop instructions. However, in certain embodiments, inresponse to the predicted number of iterations exceeding a specifiedsecond threshold, the processor initiates the loop mode without waitingfor the first threshold number of iterations of the loop to be executed,thereby conserving processor resources by entering the loop mode soonerthan in other embodiments of use of the loop mode.

FIG. 1 is a block diagram of an instruction pipeline architecture withina processor 100 implementing LEP in accordance with some embodiments.Only a few components of the processor 100 are illustrated for sake ofsimplicity of illustration. Further, certain components of the processor100 may be considered part of either a front side or a back side of theprocessor 100 as conventionally understood, for retrieving and executinginstructions, respectively, but are not so designated herein as thetechniques described herein are applicable to a plurality of types ofprocessors having various components, architectures, instruction sets,modes of operation, and so forth. The processor 100 generally executessets of instructions (e.g., computer programs) to carry out tasks onbehalf of an electronic device. Accordingly, in some embodiments, theprocessor 100 is incorporated into an electronic device such as adesktop computer, laptop computer, server, smartphone, game console,household appliance, and the like.

To support execution of instructions, the processor 100 includes aninstruction pipeline 114 including an instruction cache 101, a datacache 102, an instruction fetch unit 103 having one or more predictors104, a loop exit predictor 105, a decoder 106, a reorder buffer 107,registers 108, a loop instruction buffer 109, reservation stations 110,a load/store unit 111, one or more execution units 112, and a powercontroller 117. The instruction pipeline 114 is operates in at least twomodes: an active (non-loop) mode and a loop mode. In the active mode,the components of the processor 100 are provided with power for activeexecution of instructions. In the loop mode, the processor 100 placesone or more components in a low-power state to conserve one or moreresources including energy that would have been consumed in the activemode such as while loop instructions are repeatedly executed whilecertain components remain idle.

In the active mode, the instruction fetch unit retrieves instructionsfrom the instruction cache 101 based on a value stored at a programcounter 113. In some embodiments, the instruction fetch unit 103 alsofetches instructions based on predictions generated by the predictors104. The predictors 104 include branch predictors and loop predictorsthat identify branching instructions, generate branch target addresses,loop instructions, and perform other branch, loop, and predictionfunctions.

The instruction fetch unit 103 provides the fetched instructions to thedecoder 106, which converts each instruction to one or moremicro-operations (micro-ops). A dispatch stage (not shown) of thedecoder 106 sends each micro-op to a corresponding one of the load/storeunit 111 and execution units 112 for execution. The reorder buffer 107manages the scheduling of execution of the micro-ops at the load/storeunit 111 and the execution units 112. In addition, the reservationstations 110 manage access to the registers 108 by the load/store unit111 and execution units 112. After execution of the correspondingmicro-operations, each instruction is retired at a retire stage (notshown) of the instruction pipeline 114.

In the loop mode, the instruction pipeline 114 executes iterations of aloop using the loop instruction buffer 109. As used herein, a loop is aset of instructions that is repeatedly executed until a conditionalbranch terminating the loop is taken. For example, for some loops, theconditional branch instruction is a relative jump instruction thatincludes an offset that is added to the program counter 113 pointing tothe conditional branch instruction. In some embodiments, in order to beidentified as a loop, the instruction pipeline 114 identifies that theconditional branch instruction was taken a threshold number of times(e.g., 2, 3, 4, 5) in the most recent execution instance of the loop. Aniteration of a loop refers to a single execution of the instructions ofthe loop.

Returning to the loop mode, in response to detecting an instruction loop(e.g., based on logic of the predictors 104 indicating an instructionloop), the instruction pipeline 114 stores one or more of the micro-opsfor the instructions of the loop in the loop instruction buffer 109. Inthe loop mode, the loop instruction buffer 109 repeatedly provides themicro-operations to the load/store unit 111 and the execution units 112for execution until the loop exit is reached. Thus, in the loop mode,the instruction fetch unit 103 suspends retrieving instructions from theinstruction cache 101. When in the loop mode, certain components of theprocessor 100 including one or more of the components of the instructionpipeline 114 are placed in a low-power mode or state by the powercontroller 117 to conserve power as illustrated by a dashed line 118.For example, the power controller 117 powers down the instruction fetchunit 103, one or more predictors 104, the loop exit predictor 105, andthe decoder 106 while maintaining other components in an active state,such as the loop instruction buffer 109, the load/store unit 111, andthe execution units 112. While in the active state, certain componentsremain powered on and perform their functions until a loop exitcondition occurs and power is restored to those components that wereplaced in the low-power mode (e.g., before, during, or after enteringloop mode).

To support efficient execution of the loop mode, the instructionpipeline 114 includes a loop exit predictor (LEP) 105 that predicts thenumber of iterations of each executed loop. To illustrate, the LEP 105stores a loop history 116 that indicates patterns in loops executed atthe instruction pipeline 114. In some embodiments, the LEP 105 generatesand stores the loop history 116 during one or more dedicated trainingperiods of the instruction pipeline 114. During each training period theinstruction pipeline 114 executes specified sets of instructions, countsthe number of iterations of each executed loop, and stores the number ofiterations at a storage structure designated to predict the number ofloops 115. In some embodiments, during normal operation of the processor100, the instruction pipeline 114 continues to count iterations of eachexecuted loop and based on the iterations adjusts the predicted numberof loops 115.

In some embodiments, the LEP 105 supports efficient use of the loop modein a number of ways. For example, for some loops have relatively fewiterations, the resource cost of entering and exiting the loop modeexceeds the resource savings for using the loop mode. Accordingly, insome embodiments the instruction pipeline 114 employs the predictions ofthe LEP 105 to identify loops predicted to have relatively fewiterations, and to avoid entering loop mode for those loops. Thus, inresponse to the predicted number of loops 115 for a loop being lowerthan a threshold, the instruction pipeline 114 prevents entry into theloop mode.

Further, for loops having a relatively high number of iterations,resource conservation is enhanced by entering the loop mode morequickly, such that more iterations of the loop are executed in the loopmode. Accordingly, in some embodiments, the instruction pipeline 114employs the predictions of the LEP to identify loops predicted to have arelatively high number of iterations, and accelerates entry into theloop mode for those loops. Thus, in response to the predicted number ofloops 115 for a loop being higher than a threshold (e.g., a firstthreshold), the instruction pipeline 114 enters the loop mode for thefirst iteration of the loop.

In other embodiments, the instruction pipeline 114 uses the LEP 105during the loop mode itself. This use can be better understood withreference to FIG. 2. FIG. 2 is a block diagram of an alternativeconfiguration of the processor 100 whereby the instruction pipeline 114maintains the loop exit predictor 105 in an active state during the loopmode (as illustrated by the placement of the LEP 105 relative to thedashed line 218) in accordance with some embodiments. When in the activestate during the loop mode, the loop exit predictor 105 continues topredict the number of loop iterations. For example, the loop exitpredictor 105 updates the predicted number of loop iterations likely tobe performed by the loop being executed, and the loop exit predictor 105updates a timing of restoring power to the components of the instructionpipeline 114 that were placed in the low-power mode based on the updatedprediction so that the loop mode is exited prior to a branchmisprediction that undesirably results in a pipeline flush which is botha performance and power overhead.

To illustrate, in a conventional processor, the end of a loop, andtherefore the exiting of the loop mode, is indicated by a branchmisprediction for the branch instruction that ends the loop. However, aswith other mispredictions, the branch misprediction that indicates theend of the loop requires the instruction pipeline to be flushed and thepipeline returned to an earlier state. Thus, executing the loop untilencountering a misprediction results in a power loss by way of apipeline bubble whereby one or more downstream components such as thedecoder 106, the reorder buffer 107, the registers 108, the reservationstations 110, the load/store unit 111, and the execution units 112 arestarved for instructions. In contrast, the loop exit predictor 105 ismaintained in the active state and predicts the exit to the loop. Inresponse to the predicted exit, the instruction pipeline exits the loopmode by returning the instruction fetch unit 103 and other modules to anactive state. The instruction pipeline 114 thereby avoids a branchmisprediction for the loop exit and thus avoids a mispredict performancepenalty.

FIG. 3 is a block diagram of the processor 100 of FIG. 1 illustratingadditional aspects of the LEP 105 in accordance with some embodiments.In addition to the predicted number of loops 115 and the loop history116, the loop exit predictor 105 further includes: a loop instructionbuffer 302, loop prediction logic 303, one or more loop counters 304,loop identifiers 305, a first loop threshold 306, a second loopthreshold 307, a loop prediction 308, one or more comparison results309, and one or more confidence values 310. The loop prediction logic303 provides loop exit predictions based on a set of instructions thatare identified as being executed repeatedly. A loop prediction 308includes identifying and storing the predicted number of loops for aparticular loop or set of one or more loop instructions. The loopcounters 304 and the loop identifiers 305 are used by the loop exitpredictor 105 and the instructions of the loop instruction buffer 302.For example, the loop counters 304 are used in training to identify whena set of instructions is executed as a loop, and used during loopexecution to keep track of how many iterations of loop instructions havebeen completed. A respective loop counter 304 is compared against apredicted loop exit in preparing to exit the loop at a predicted loopexit count. One or more loops may be encountered when executingprocessor instructions and the processor 100 maintains a history of aplurality of executing loops in the loop history 116 such as when asecond loop is executing inside of a first loop. The loop counters 304include at least loop confidence values, and current, past, andpredicted loop iteration values.

During a training phase, the loop exit predictor 105 detects the loopand the loop exit branch in the set of processor instructions. Trainingincludes the loop exit predictor 105 keeping track of a number of loopiterations repeatedly executed for a particular set of loop instructionssuch as in one of the loop counters 304. Whenever a particular loopiterates a same number of times as in a previous run or executioninstance of the loop, a confidence value 310 is incremented, and theconfidence value 310 is used by the loop exit predictor 105 whenproviding its estimate of the loop exit.

At a time of identification or prediction, the loop exit predictor 105searches for a matching loop identifier in a current set of the loopidentifiers 305. A hit to an LEP entry in the loop identifiers 305implies that a predicted branch instruction is an exit branchinstruction. Finding the hit in the loop identifiers 305 includesmatching a characteristic of a loop instruction to at least oneidentifier in the loop identifiers. If a current iteration of theparticular loop being tracked by the loop exit predictor 105 is equal toa total number of iterations predicted by loop exit predictor 105, thenthe particular loop is predicted to exit during this iteration. That is,the particular loop iteration of the loop exit branch is predicted asnot-taken. Otherwise, the loop exit branch is predicted to be taken.

According to certain embodiments, LEP performed by the loop exitpredictor 105 is only performed when a confidence value 310 associatedwith the particular branch is sufficiently high. If the confidence value310 is too low (i.e., fails to exceed a confidence threshold) or ifthere is no hit to an LEP entry in the loop identifiers 305, then thebranch is predicted or subjected to processing by other branchpredictors such as one of the predictors 104 of the instruction fetchunit 103. Since the loop prediction logic 303 is specifically tuned forloop exit branches, its prediction accuracy is usually higher than anaccuracy of other branch predictors or general type predictors when theprocessor 100 executes instructions of exit branches. The loopprediction logic 303 provides the loop prediction 308 for each loop. Theloop prediction 308 indicates whether a set of executing instructions isindeed a set of loop instructions. The loop exit predictor 105 providesthe predicted number of loops: a number of iterations that the set ofloop instructions is likely to complete before exiting.

According to certain embodiments, entering the loop mode is triggered bysaturating a specified number of bits of a direction history of theconditional branches (not shown) to ensure that a loop (e.g., a set ofone or more instructions) is actually being executed by the processor100. For example, a loop is identified by finding a repetitive patternalong a direction in a history register. For a direction historyregister that is 100 bits, if a group of five bits out of the 100 bitsis repeating, then that implies that there is a loop with fiveconditional branches. In operation, the loop mode is entered only aftersaturating a certain number of bits of the direction history register orexceeding a direction threshold (value). For a saturation level of 80bits to saturate, and a loop that has only two conditional branches, asystem would have to wait 40 iterations of the loop because only at thatpoint would have a direction history variable (e.g., dirHist) becomesaturated (reach 80 counted bits) thereby triggering entrance of theloop mode. On the other hand, if the number of bits to saturate is toolow (e.g., 10), then the system would have entered the loop mode rightafter a fifth iteration to reach the value of 10 by incrementingsaturation by two bits for each loop iteration. If the particular loopin this situation is only supposed to run (or predicted to run) for sixiterations, then the processor would enter the loop mode and thenimmediately come out of loop mode thereby wasting the benefits providedby the loop mode. Generally, if the number of bits of the directionhistory is greater than the direction threshold, then the processor 100is identified as executing a loop. The larger the direction threshold,the longer it takes the processor 100 to be triggered into entering theloop mode and the lower a chance to identify an opportunity to savepower by entering the loop mode when the instructions are actually loopinstructions. If the direction threshold is too low, the processor 100could enter the loop mode when no loop or an excessively short loop isactually being executed. Hence, there is a balancing as to when to enterthe loop mode in view of the length of the loop being executed. In atleast some embodiments, branch prediction includes a branch direction, adirection threshold, and a target address. The same is true for LEP suchthat LEP includes a loop direction, a loop threshold, and a loop exittarget address.

The processor 100 also uses the loop prediction 308 before and duringexecution of micro-ops to determine when to enter the loop mode and toexit the loop mode. In particular, once the processor 100 has determinedthat the micro-ops are likely executing a loop, the loop prediction 308is compared against the first loop threshold 306 and the second loopthreshold 307. The comparisons yield a respective comparison result 309,one result per comparison. Based on at least one of the comparisonresults 309, the processor 100, enters the loop mode.

When an application (e.g., software application as a source of micro-opsfor the processor 100) is going through a repetitive loop, micro-ops ofthe instructions (or instructions) pertaining to that loop are cached inthe loop instruction buffer 302 before or during the loop mode. Duringthe loop mode, the micro-ops are executed out of the loop instructionbuffer 302 by one or more cores such as the first processor core 301,and certain other components of the processor 100 are placed in alow-power mode thereby saving power that would have been consumed byoperation of the components operating at full power. For a set of loopinstructions that is too large to fit in the loop instruction buffer302, the loop exit predictor 105 remains powered up, and the loopinstruction buffer 302 is powered down to a low-power or lower-powerstate, and energy consumption by the processor 100 remains a result ofthe loop mode. In this situation, the loop exit predictor 105 remainspowered up and continues to predict the exit of the loop and a directionof the loop instructions when the instructions are pulled from theinstruction cache 101 and provided to the first processor core 301.According to at least some embodiments, the loop mode occurs when one ormore components are powered down or placed into a low-power mode andwhile loop instructions are executed such as from the loop instructionbuffer 302.

When the predictors 104 are powered down or placed into a low-power modein the loop mode, one way to exit the loop mode is to have aninstruction execution component send a redirect message, an exit signal,indicating that the exit branch was mispredicted to one or morecomponents of the processor 100. The exit signal causes the instructionpipeline 114 to fetch and execute instructions that occur after theloop. Because branch mispredicts are expensive in terms of wasted powerand wasted execution cycles, an improperly selected or designateddirection threshold comes with a power performance overhead. Hence,there is a trade-off to be made in terms of obtaining a power savings byentering the loop mode versus a power performance overhead formispredicting the exit branch instruction. For short loops (e.g., loopsless than 5 iterations, loops less than 10 iterations), in somesituations the power performance overhead of the mispredicted exitbranch outweighs the power savings in the loop mode for a particularconfiguration of the processor 100. Another way to exit the loop modeinvolves the loop exit predictor 105 remaining powered up and the loopexit predictor 105 providing the loop exit signal upon successful loopexit prediction. In this way, a mispredict is avoided by having theinstruction pipeline 114 timely deliver for execution instructionsoccurring after the loop.

FIG. 4 is a flow diagram illustrating a method 400 for implementing loopexit prediction for a relatively large loop iteration prediction inaccordance with some embodiments. The method 400 is performed bycomponents of a processor such as the components of the processor 100.At block 401, the method 400 includes identifying whether a branchinstruction is a loop instruction—a loop to potentially be executed inthe loop mode. If so, at block 402, the processor determines a loopidentifier and a number of loop iterations for the loop. Thisidentification includes looking up the loop identifier in a set ofstored loop identifiers such as the loop identifiers 305. At block 403,the processor determines whether the determined number of loopiterations exceeds a first loop threshold such as the first loopthreshold 306. For example, the first loop threshold is a relativelylarge number (e.g., 500; 1,000; 10,000) for use in identifying a loop asa large loop having a relatively large number of predicted loopiterations to be executed by the processor. If the determined number ofloop iterations exceeds the first loop threshold, the loop mode isdirectly entered. Further, according to some embodiments, if the firstloop threshold is exceeded, no check is made whether a certain directionhistory threshold or direction history variable is exceeded: the loopmode is directly entered without making this check.

If the determined number of loop iterations does not exceed the firstloop threshold, at block 404, the processor determines whether thedetermined number of loop iterations exceeds a second loop thresholdsuch as the second loop threshold 307. For example, the second loopthreshold is a relatively small number (e.g., 15, 10, 5, 3) for use inidentifying a loop as a small loop having a relatively few number ofpredicted loop iterations to be executed by the processor. If thepredicted number of loop iterations does not exceed the secondthreshold, at block 405, the processor waits for identification of anext loop by maintaining one or more components of an instructionpipeline in an active mode including maintaining the components in apowered up state, and execution returns to block 401. In this situation,the processor and loop exit predictor have encountered a loop that islikely too small to benefit from power savings of the loop mode and theprocessor avoids entering the loop mode based on the determinationrelative to the first loop threshold and the second loop threshold.Alternatively, the processor avoids entering the loop mode based on thedetermination relative to the second loop threshold.

If the determined number of loop iterations does not exceed the firstloop threshold and exceeds the second threshold, at block 406, theprocessor waits for a certain number of actual loop iterations beforeconfirming that the instructions are executing within the loop. If thedetermined number of loop iterations exceeds the first threshold atblock 403—or after waiting the certain number of successful loopexecutions at block 406—the method 400 continues at block 407 at which aset of loop instructions are stored in a loop buffer, such as the loopbuffer 109. Subsequently, the loop instructions are repeatedly executedfrom the loop buffer. At block 408, one or more components of theprocessor are placed in a low power mode. At block 409, the loopinstructions are executed until a branch misprediction occurs or theloop instructions are executed for the number of predicted loopiterations and exited by having the loop exit predictor accuratelypredict the loop exit and provide the loop exit signal. In thissituation, no pipeline bubble is encountered by the processor. Uponexiting the loop, at block 410, power is restored to the processorcomponents that were placed in the low-power mode during the loop modeat block 408. Once power is restored, the processor waits for a nextloop at block 405.

FIG. 5 is a flow diagram illustrating a method 500 for implementing loopexit prediction for a relatively small loop iteration prediction inaccordance with some embodiments. The method 500 is performed bycomponents of a processor such as the processor 100. At block 501, themethod 500 includes predicting a number of loop iterations associatedwith a set of loop instructions. In response to the predicted number ofloop iterations exceeding a first loop iteration threshold at block 502,the set of loop instructions are executed in a loop mode and, inresponse to the predicted number of loop iterations failing to exceedthe first loop iteration threshold (e.g., the predicted number of loopiterations is less than or equal to the loop iteration threshold), theset of instructions are operated in an active mode. In particular, for apositive outcome at block 502, at block 503 the loop mode includesplacing at least one component of an instruction pipeline of theprocessor into a low-power mode or state. Further, according to someembodiments, at block 503, no check is made whether a certain directionhistory threshold or direction history variable is exceeded: the loopmode is entered directly upon determining that the predicted number ofloop iterations exceeds the first loop iteration threshold. At block504, the loop mode also includes executing the set of loop instructionsfrom a loop buffer.

At blocks 505-507, the loop mode includes certain additional steps inaccordance with some embodiments of the method 500. For example, atblock 505, the loop mode updates the predicted number of loop iterationsassociated with the set of loop instructions. The predicting and theupdating of the number of loop iterations is performed by a loop exitpredictor such as the loop exit predictor 105. At block 506, the loopmode determines a time to restore power to the components of theinstruction pipeline of the processor that were placed in the low-powermode. The time to restore power to the low-powered components is able tocome before an end of execution of the loop instructions since a leadtime (e.g., a certain number of clock cycles) is often needed to fillthe instruction pipeline with instructions that come sequentially afterexiting the loop to avoid a pipeline bubble. At block 507, the loop modepredicts an exit for the set of loop instructions. The processordetermines the time to restore power to the components placed in thelow-power mode and determines a next instruction address based on thepredicted exit.

At block 508, the active mode of the method 500 includes maintaining theat least one component of the instruction pipeline in a powered upstate. For example, a loop exit predictor such as loop exit predictor105 is maintained with power. At block 509, the active mode alsoexecutes the set of loop instructions from an instruction fetch stageunit of the instruction pipeline. For the method 500, for each loop, theprocessor is operating either in the loop mode or in the active mode.

In some embodiments, certain aspects of the techniques described abovemay implemented by one or more processors of a processing systemexecuting software. The software includes one or more sets of executableinstructions stored or otherwise tangibly embodied on a non-transitorycomputer readable storage medium. The software can include theinstructions and certain data that, when executed by the one or moreprocessors, manipulate the one or more processors to perform one or moreaspects of the techniques described above. The non-transitory computerreadable storage medium can include, for example, a magnetic or opticaldisk storage device, solid state storage devices such as Flash memory, acache, random access memory (RAM) or other non-volatile memory device ordevices, and the like. The executable instructions stored on thenon-transitory computer readable storage medium may be in source code,assembly language code, object code, or other instruction format that isinterpreted or otherwise executable by one or more processors.

Note that not all of the activities, components, or elements describedabove in the general description are required, that a portion of aspecific activity or device may not be required, and that one or morefurther activities may be performed, or elements included, in additionto those described. Still further, the order in which activities arelisted are not necessarily the order in which they are performed. Also,the concepts have been described with reference to specific embodiments.However, one of ordinary skill in the art appreciates that variousmodifications and changes can be made without departing from the scopeof the present disclosure as set forth in the claims below. Accordingly,the specification and figures are to be regarded in an illustrativerather than a restrictive sense, and all such modifications are intendedto be included within the scope of the present disclosure.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any feature(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature of any or all the claims. Moreover, the particular embodimentsdisclosed above are illustrative only, as the disclosed subject mattermay be modified and practiced in different but equivalent mannersapparent to those skilled in the art having the benefit of the teachingsherein. No limitations are intended to the details of construction ordesign herein shown, other than as described in the claims below. It istherefore evident that the particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope of the disclosed subject matter. Accordingly, the protectionsought herein is as set forth in the claims below.

What is claimed is:
 1. A method comprising: in a processor, predicting anumber of loop iterations expected to be executed for a loop associatedwith a set of loop instructions; and in response to the predicted numberof loop iterations exceeding a first loop iteration threshold, executingthe set of loop instructions in a loop mode, including: placing at leastone component of an instruction pipeline of the processor in a low-powermode; and executing the set of loop instructions from a loop buffer. 2.The method of claim 1, further comprising: in response to the predictednumber of loop iterations being less than a second loop iterationthreshold, delaying entry into the loop mode until the threshold numberof loop iterations has been executed.
 3. The method of claim 2, furthercomprising: in response to the predicted number of loop iterations beinggreater than the second loop iteration threshold, entering the loop modebefore the threshold number of loop iterations has been executed.
 4. Themethod of claim 1, wherein placing the at least one component of theinstruction pipeline in the low-power mode includes placing a loop exitpredictor of the processor in the low-power mode.
 5. The method of claim1, further comprising: after placing the at least one component of theinstruction pipeline in the low-power mode, updating, by a loop exitpredictor, the predicted number of loop iterations associated with theset of loop instructions; and determining a timing of restoring power tothe at least one component of the instruction pipeline in the low-powermode based on the updated number of loop iterations.
 6. The method ofclaim 1, further comprising: prior to predicting the number of loopiterations, identifying instructions as the set of loop instructions bymatching a characteristic of a loop instruction to an identifier in aset of stored loop identifiers.
 7. The method of claim 1, wherein theplacing of the at least one component of the instruction pipeline in thelow-power mode is performed prior to executing an instruction of the setof loop instructions.
 8. The method of claim 1, further comprising:predicting an exit for the set of loop instructions during the loopmode.
 9. A method comprising: in a processor, in response to predictinga number of loop iterations expected to be executed for a loopassociated with a set of loop instructions: storing the set of loopinstructions in a loop buffer; placing a component of an instructionpipeline of the processor in a low-power mode; executing the set of loopinstructions from the loop buffer; predicting a loop exit by a loop exitpredictor of the processor; and restoring power to the component placedin the low-power mode based on the predicted loop exit.
 10. The methodof claim 9, further comprising: comparing the predicted number of loopiterations with a first loop iteration threshold prior to powering downthe component of the instruction pipeline.
 11. The method of claim 9,wherein the component of the instruction pipeline is placed in thelow-power mode prior to executing the set of loop instructions from theloop buffer.
 12. The method of claim 9, wherein powering down thecomponent of the instruction pipeline of the processor includes poweringdown a loop exit predictor of the processor.
 13. A processor comprising:an instruction cache having a set of loop instructions for theprocessor; a loop buffer configured to store the set of loopinstructions; a loop exit predictor configured to predict a number ofloop iterations expected to be executed for a loop associated with theset of loop instructions; and wherein the processor is configured to: inresponse to the predicted number of loop iterations exceeding a firstloop iteration threshold, execute the set of loop instructions in a loopmode, including to: place at least one component of an instructionpipeline of the processor in a low-power mode; and execute the set ofloop instructions from the loop buffer; and in response to the predictednumber of loop iterations being less than or equal to the first loopiteration threshold, execute the set of loop instructions in a non-loopmode, including to: maintain the at least one component of theinstruction pipeline in an active state; and execute the set of loopinstructions fetched from the instruction cache by an instruction fetchunit of the instruction pipeline.
 14. The processor of claim 13, furthercomprising: a decoder for decoding the loop instructions intomicro-operations for execution by a functional unit of the processor,and wherein the instruction fetch unit is configured to provide the loopinstructions to the decoder from the instruction cache.
 15. Theprocessor of claim 14, wherein the instruction fetch unit is configuredto provide loop instructions to the loop exit predictor.
 16. Theprocessor of claim 13, wherein the at least one component of theinstruction pipeline placed in the low-power mode is an instructionfetch component of the processor.
 17. The processor of claim 13, whereinthe at least one component of the instruction pipeline placed in thelow-power mode is the loop exit predictor.
 18. The processor of claim13, wherein the loop exit predictor is further configured to: afterplacing the at least one component of the instruction pipeline in thelow-power mode, update the number of loop iterations associated with theset of loop instructions; and wherein a timing of restoring power to theat least one component of the instruction pipeline placed in thelow-power mode is based on the updated number of loop iterations. 19.The processor of claim 13, further comprising: a buffer of stored loopidentifiers; and wherein the loop exit predictor is further configuredto: match a characteristic of the set of loop instructions to anidentifier in the buffer of stored loop identifiers.
 20. The processorof claim 13, wherein placing the at least one component of theinstruction pipeline in the low-power mode is performed prior toexecuting any instruction associated with the set of loop instructionsand after predicting the number of loop iterations associated with theset of loop-instructions.